Microdialysis is a relatively new sampling technique designed for the in vivo analysis of endobiotic/xenobiotic substances. The microdialysis probe is surgically implanted into a vein or a tissue of interest in an animal. Substances within the bloodstream or tissue with Mr below the molecular weight cutoff of the dialysis membrane dialyze across the membrane into the perfusate. We have interfaced this technique-with mass spectral analysis using the coaxial continuous flow fast atom bombardment (CF-FAB) approach we have developed and tandem mass spectrometry. In our initial experiments, we have investigated the half-life of the suspected carcinogen tris (2-chloroethyl)phosphate in the blood of rats. Tris organophosphates have been widely used in plastics and synthetic fibers as flame retardants. We have been able to determine that the half-life of this compound in blood follows a two compartment model. The alpha half-life (due to distribution and elimination) is 2.9 min. while the beta half-life (due to elimination only) is 23 min. There was no statistical difference (T-test) between these results and those of HPLC analyses using radiotracers. As an indication of the sensitivity of the technique, after one hour, the tris concentration in the blood is approximately 20 micromolar. The signal-to-noise ratio of the MS/MS signal at this time was greater than 10:1.